DISTRIBUTION OF NITROGEN 99 



After standing for at least forty-eight hours they are filtered off 

 and washed in the following manner : 



A 3-inch Buchner funnel is covered with a hardened filter paper of 

 such a size that it fits against the bottom and side walls ; the portion 

 of the paper against the side walls is folded into about twenty plaits 

 so that it fits snugly all round. 



The precipitate is poured into this pocket and the mother liquor 

 removed by suction and by pressing down the precipitate with a 

 flattened rod. 



The filtrate is returned to a beaker. 



Washing is effected with 10-12 c.c. of a solution containing 2*5 

 grams of phosphotungstic acid and 3*5 grams of hydrochloric acid per 

 100 c.c. ; this is first used to dislodge the particles remaining in the 

 flask ; it is then poured upon the precipitate which is stirred up until 

 all lumps are broken and until there is only a granular suspension. 

 It is then sucked dry as before. The washing is repeated three to 

 four times in this manner. Then the precipitate on the filter is 

 washed five to ten times with the same solution from a wash bottle, 

 commencing round the edges and sucking dry each time. 



It frequently happens that the later washings run through some- 

 what turbid ; these are filtered through a folded paper. All washings 

 are combined with the main filtrate. The washing must be continued 

 till the liquid is free from calcium ; I c.c. of the filtrate when tested 

 with oxalic acid in 3 per cent, sodium hydrate must give no turbidity 

 even after standing for several minutes. 



The filtration and washing may be effected equally well on a 

 2-inch Buchner funnel with a hardened filter paper pressed over the 

 perforations in the usual manner, as was shown by Hartley [1914] and 

 by Van Slyke [1915, i]. A steady and moderately strong suction is 

 continued without interruption during the filtration and washing. 

 The 10-15 c.c. of washing solution 1 are poured upon the precipitate, 

 which is stirred up with each portion. The washing is completed as 

 soon as a few drops of the liquid on being allowed to flow upon a 

 10 per cent, solution of sodium hydrate containing sodium oxalate 

 and on gently shaking to bring the liquids in contact show no cloudi- 

 ness even after standing for several minutes. 



If there be any doubt as to the purity of the phosphotungstates 

 they may be recrystallised by suspending in water and dissolving by 

 adding sodium hydrate. The solution is acidified, heated, and the 



1 This should be cooled to o. 



7* 



